<html><head><meta http-equiv="Content-Type" content="text/html charset=utf-8"></head><body style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space;" class="">OK. I see that the Mach-O linker is not even built when LLD is enabled in Release_40, only the PE/COFF and ELF linkers are built.<div class=""><br class=""></div><div class="">From looking at reviews it appears that Clang was able to be linked with LLD on Darwin about 2 years ago, so Mach-O support seems to have regressed.</div><div class=""><br class=""></div><div class="">Curious as to pointers to primordial branches with whatever needs to be resurrected. I couldn’t find any Mach-O cmake flags to enable its build. A pointer to a branch or tag that might have a working Mach-O LLD would be a start.<br class=""><div class=""><br class=""><div><blockquote type="cite" class=""><div class="">On 7 Jun 2017, at 11:38 AM, Michael Clark <<a href="mailto:michaeljclark@mac.com" class="">michaeljclark@mac.com</a>> wrote:</div><br class="Apple-interchange-newline"><div class=""><meta http-equiv="Content-Type" content="text/html charset=utf-8" class=""><div style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space;" class="">Hi Rui,<div class=""><br class=""></div><div class="">The motivation would be primarily that LLVM/Clang/LLD are community projects such that if I or someone in the community added support for e.g. symbol aliases, then it could be reviewed and potentially merged. ld64 on the other hand does not have a community process for patch submission and code review that I am aware of so its unlikely that if someone from the community came up with a patch to support aliases that it would be merged.</div><div class=""><br class=""></div><div class="">In that case I might check out the LLD code and try linking “x86_64-xnu-musl” with it. My requirements are likely simpler than Apple’s however I do need symbol aliases and these are not supported by ld64. The linker synthesised symbols are likely not too difficult to add if they are not present… now on my to do list…</div><div class=""><br class=""></div><div class="">Michael.</div><div class=""><br class=""><div class=""><blockquote type="cite" class=""><div class="">On 7 Jun 2017, at 11:30 AM, Rui Ueyama <<a href="mailto:ruiu@google.com" class="">ruiu@google.com</a>> wrote:</div><br class="Apple-interchange-newline"><div class=""><div dir="ltr" class="">Hi Michael,<div class=""><br class=""></div><div class="">The Mach-O version of LLD is not being developed actively, and if some feature is missing, it is likely that it's just not implemented. What is your motivation to use LLD instead of ld64?</div></div><div class="gmail_extra"><br class=""><div class="gmail_quote">On Tue, Jun 6, 2017 at 4:08 PM, Michael Clark via llvm-dev <span dir="ltr" class=""><<a href="mailto:llvm-dev@lists.llvm.org" target="_blank" class="">llvm-dev@lists.llvm.org</a>></span> wrote:<br class=""><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><div style="word-wrap:break-word" class=""><div class="">Hi Folks,</div><div class=""><br class=""></div><div class="">I have a question regarding LLD support for ld64 mach-o linker synthesised symbols. I did a quick search of the LLD source and I can not find support for them so before I start trying to use lld I thought I would ask.</div><div class=""><br class=""></div><div class="">I have found a couple of cases where they are essential. i.e. where there is no other way to get the required information, such as getting the address of the mach-o headers of the current process, with ASLR enabled, if the process is not dyld as exec on macOS only provides the mach header address to dyld (*1). They are used inside of dyld and I am now using them in “x86_64-xnu-musl”.</div><div class=""><br class=""></div><div class="">It’s possible to resolve a mach-o segment offset or a mach-o section offset using these special ld64 linker synthesised symbols. See resolveUndefines:</div><div class=""><br class=""></div><div class="">- <a href="https://opensource.apple.com/source/ld64/ld64-274.2/src/ld/Resolver.cpp.auto.html" target="_blank" class="">https://opensource.apple.<wbr class="">com/source/ld64/ld64-274.2/<wbr class="">src/ld/Resolver.cpp.auto.html</a></div><div class=""><br class=""></div><div class="">There are 4 special symbol prefixes for the mach-o linker synthesised symbols:</div><div class=""><br class=""></div>- segment$start$__SEGMENT<br class="">- segment$end$__SEGMENT<br class="">- section$start$__SEGMENT$__<wbr class="">section<br class=""><div class="">- section$end$__SEGMENT$__<wbr class="">section</div><div class=""><br class=""></div><div class="">In asm:</div><div class=""><br class=""></div><blockquote style="margin:0 0 0 40px;border:none;padding:0px" class=""><div class="">/* get imagebase and slide for static PIE and ASLR support in x86_64-xnu-musl */</div><div class=""><br class=""></div><div class="">.align 3</div><div class="">__image_base:</div><div class="">.quad segment$start$__TEXT</div><div class="">__start_static:</div><div class="">.quad start</div><div class="">.text</div><div class="">.align 3</div><div class="">.global start</div><div class="">start:</div><div class=""> xor %rbp,%rbp</div><div class=""> mov %rsp,%rdi</div><div class=""> andq $-16,%rsp</div><div class=""> movq __image_base(%rip), %rsi</div><div class=""> leaq start(%rip), %rdx</div><div class=""> subq __start_static(%rip), %rdx</div><div class=""> call __start_c</div></blockquote><div class=""><br class=""></div><div class="">In C:</div><div class=""><br class=""></div><blockquote style="margin:0 0 0 40px;border:none;padding:0px" class=""><div class="">/* run C++ constructors in __libc_start_main for x86_64-xnu-musl */</div><div class=""><br class=""></div></blockquote><blockquote style="margin:0 0 0 40px;border:none;padding:0px" class="">typedef void (*__init_fn)(int, char **, char **, char **);<br class="">extern __init_fn __init_start __<wbr class="">asm("section$start$__DATA$__<wbr class="">mod_init_func");<br class="">extern __init_fn __init_end __asm("section$end$__DATA$__<wbr class="">mod_init_func”);<br class=""><div class=""><br class=""></div><div class="">static void __init_mod(int argc, char **argv, char **envp, char **applep)</div><div class="">{</div><div class=""> for (__init_fn *p = &__init_start; p < &__init_end; ++p) {</div><div class=""> (*p)(argc, argv, envp, applep);</div><div class=""> }</div><div class="">}</div></blockquote><div class=""><br class=""></div><div class="">Michael.</div><div class=""><br class=""></div><div class="">[1] <a href="https://github.com/opensource-apple/xnu/blob/dc0628e187c3148723505cf1f1d35bb948d3195b/bsd/kern/kern_exec.c#L1072-L1111" target="_blank" class="">https://github.com/<wbr class="">opensource-apple/xnu/blob/<wbr class="">dc0628e187c3148723505cf1f1d35b<wbr class="">b948d3195b/bsd/kern/kern_exec.<wbr class="">c#L1072-L1111</a></div></div><br class="">______________________________<wbr class="">_________________<br class="">
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